1. Field of the Invention
The present invention relates to a flat display display device, and more particularly, to an organic electro-luminescent display device and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for effectively protecting the organic electro-luminescent device from the oxygen or the moisture.
2. Discussion of the Related Art
With the advent of the information age of high technology due to the development of information communication and computers, many display devices are extensively used. A cathode ray tube (CRT) which makes a fluorescent body to be luminescent by emitting electrons from an electron gun, such as a television, is a display for a desk top computer, and about one hundred million cathode ray tubes are in demand each year. A liquid crystal display (LCD) device for a notebook computer is also used for a monitor or a digital camera. The LCD device is not a spontaneous emitting device, so that it reproduces an image by the light of a backlight. However, the CRT and EL device are spontaneous emitting types. Especially, the EL device is divided into an organic EL type and an inorganic EL type according to the fluorescent compound used for an emitting layer.
Further, the inorganic EL device is divided into a dispersion type and a thin film type, which are all accelerated since electrons in the fluorescent body is located below the high electric field. Accordingly, a collision is generated at the center of the emission. The currently used inorganic EL is almost operated by an alternating current, and brightness is dependent on voltage and frequency. In the meantime, the organic EL device injects electrons and holes from the outside, and emits light by the re-combination energy of the electrons and the holes.
When applied as a display, an organic EL device has a wider viewing angle, a higher contrast, and more excellent visibility than an LCD device, since the organic EL device is a spontaneous emitting type device. Also, since the backlight is not required, the organic EL device can be fabricated in slim and light size. The organic EL device can be operated with a low voltage of direct current, so that a moving picture can be displayed due to a fast response speed. Nowadays, the organic EL device is in a spotlight for the IMT-2000 display, and has a wide operation temperature range, since it is solid and has a strong vibration resistance. Especially, in the organic EL device, a current is applied only to pixels requiring an emission. Accordingly, as compared to the LCD device, which always has to turn on the backlight on the entire surface, the organic EL device has a much lower power consumption regardless of the display contents.
The organic EL display device has a cathode electrode and an anode electrode facing into each other on a transparent substrate, such as a glass substrate. By applying a voltage between the cathode electrode and the anode electrode, a light is emitted from an organic emitting layer. At this time, the anode electrode is formed by indium-tin-oxide (ITO) having an excellent conductivity and a light transmissivity, so that the light emitted from the organic emitting layer can be transmitted well and holes can be easily supplied. Also, the cathode electrode is formed by a metal haying a low work function, so that electrons can be easily supplied.
Accordingly, if (+) voltage and (−) voltage are respectively applied to the anode electrode and the cathode electrode, the holes injected from the anode electrode and the electrons injected from the cathode electrode are re-combined in the organic emitting layer, thereby emitting light.
In the meantime, the organic luminescent device has a plurality of unit pixels arranged as a matrix form, and selectively drives the organic emitting layer of the unit pixels by a thin film transistor provided at each unit pixel, thereby displaying an image.
The organic emitting layer of the organic luminescent device is composed of a hole transport layer, an electron transport layer, and an organic emitting layer formed between the hole transport layer and the electron transport layer. At this time, the organic layer is very sensitive to moisture, oxygen in the air, and temperature, and if contacted to the moisture and oxygen, the structure is changed, thereby degrading emitting efficiency.
Accordingly, an encapsulation process for forming a packaging plate is performed to isolate the organic luminescent device from the moisture and oxygen.
FIGS. 1 and 2 are structures of an organic EL display device in accordance with the related art.
A related art organic EL display device comprises a thin film transistor TFT, an organic electro-luminescent device, and a capacitor (Cst).
More specifically, the thin film transistor TFT comprises an active layer 120, a gate electrode 130, source and drain electrodes 160 and 150 at source and drain regions 123B and 123A formed at both sides of the active layer 120, and a gate insulating layer 123 formed between the gate electrode 130 and the active layer 120 for insulation therebetween. And, a barrier layer 101 is formed on the entire surface of the glass substrate 100 for preventing a foreign substance from adhering to the TFT from the substrate 100, a passivation layer 128 including the source/drain electrodes 160 and 150, and second and third interlayer 127 and 129 is formed on the TFT.
The electro-luminescent device comprises a transparent electrode 170 electrically connected to the drain electrode 150 of the TFT, an organic emitting layer 172 formed on the transparent electrode 170 and having a hole transport layer, an emitting layer, and an electron transport layer, and an opaque electrode 175 formed on the organic emitting layer 172.
The capacitor Cst comprises a power line 140, a polycrystalline silicon layer 121, and a first interlayer 125 between the power line 140 and the polycrystalline silicon layer 121. Also, a sealant 108 is formed at the outer skirt of the substrate on which the TET, the organic luminescent device, and the capacitor are formed, and by the sealant 108, a packaging plate 107 is attached to the substrate 100.
The packaging plate 107 shields moisture and oxygen penetrated into the organic EL device from the outside, and has an absorbent 106 attached by a semi-transmitting film 105 therein.
The absorbent 106 of BaO, CaCO3, CaO, InO, zeolite, silicagel, alumina, and so on, is exposed to moisture and then removes the moisture by a chemical reaction. The sealant 108 of epoxy resin seals the glass substrate 100 and the packaging plate 107.
Also, as a packaging plate, a metal plate 107 can be used, as shown in FIG. 1. Alternatively, a glass substrate 111a can be used, as shown in FIG. 2. In case of using the metal plate 107 as a packaging plate, if the area of the metal plate in which the absorbent has to be mounted becomes wide, it is hard to evenly form the metal plate. Also, in order to evenly form the metal plate, the metal plate has to be thick, and if the thickness of the metal plate is increased, the device becomes heavy and large.
Also, in case of using a glass substrate as a packaging plate, the glass substrate has to be cut concavely towards an inner direction in order to mount the absorbent. However, the cutting depth of the glass substrate is limited due to a mechanical strength thereof, so that mounting the absorbent is improper. Also, in case of using a thick glass substrate in order to improved the mechanical strength, the organic EL display device is increased in weight and size.
In order to resolve such problems and to reduce a production cost, a hermetic sealing method in which a passivation layer is used as the packaging plate has been researched. The passivation layer used as the packaging plate has to be transparent, humidity resistant, heat resistant, and low in stress. Also, the passivation layer has to be formed at a low temperature. To meet such characteristics, one of an inorganic layer, an organic layer, and hybrid is used as the packaging plate. However, in case of the inorganic layer, the formation rate of a thin film is slow, thereby degrading productivity. Also, since a stress is high, it is increased as the layer becomes thick. Accordingly, the internal structure of the layer is changed, so that moisture can be penetrated into the layer.
Also, in case of the organic layer, even if the organic layer has advantages of an easy layer formation and a lower stress, moisture can exist after the layer formation. Accordingly, a complete moisture shielding is difficult when only the organic layer is used.
Also, such a single layer has a pinhole, through which the moisture or oxygen is penetrated.
Therefore, in the related art, a hybrid structure, in which the organic layer and the inorganic layer are deposited, is used.
FIG. 3 shows an organic EL display device in which a passivation layer of the hybrid structure is applied in accordance with the related art.
As shown in FIG. 3, a conventional packaging layer 180 is formed by combining an organic layer and an inorganic layer. In case of applying a hybrid structure, pinholes 181 formed at each layer are conflicting, so that a penetration path of moisture and oxygen is lengthened to preserve a life span of the organic luminescent device.
However, the hybrid structure cannot completely shield the moisture penetration, since is it difficult to maintain the stress balance of a thin film due to the mixture of the organic layer and the inorganic layer, and the substrate is bent due to the stress. The bent substrate causes an opening of the lines formed on the substrate, such as a gate line, a data line, a power line and so on.